Move contributing link to wiki.

We're adding additional information on contributing to the wiki so
link there from CONTRIBUTING.md. The file is retained so that GitHub
will still link to it for all incoming pull requests.

CONTRIBUTING.md

@@ -1,13 +1,11 @@
 # Contributing
 
-General information on contributing to dotnet projects is in the [Contributing Guide]. 
-This document contains information about coding styles, source structure, making
+See [Contributing] on the wiki for information about coding styles, source structure, making
 pull requests, and more.
 
-[Contributing Guide]: https://github.com/Microsoft/dotnet/blob/master/CONTRIBUTING.md
-
+[Contributing]: https://github.com/dotnet/corefx/wiki/Contributing
 # Developers
 
-For details about developing in this repo see the [Developer Guide].
+See the [Developer Guide] for details about developing in this repo.
 
 [Developer Guide]: https://github.com/dotnet/corefx/wiki/Developer-Guide